Geological Survey of Denmark and Greenland Bulletin 11 ... - GEUS

Fig. 4. The gold bearing mylonite zone. K-
feldspar occurs on the right side of the
yellow magnet pen (10 cm long). The zone
also contains pyrite, chalcopyrite and
magnetite.
weathering gneiss and amphibolite (Fig. 3). At the western
border of the tract a gold-bearing shear/mylonite zone
follows the contact between brown gneisses and amphibolites.
The gold-bearing shear/mylonite zone (Fig. 4) is
invaded by pegmatite sheets as well as centimetre-wide
veins consisting of red alkali-feldspar and quartz with occasional
pyrite and magnetite. The estimated relative volume
of pegmatite in the tract varies from 1 to 10% (Stendal
et al. 2002, 2004).
The most promising gold showings are found in a coastal
profile along the shear/mylonite zone, which can be
followed along strike for several hundreds of metres (Figs
1, 2). The studied site is a cliff exposure consisting of
mylonite (Fig. 4) and a rusty weathered band (10–20 cm
in width) mineralised with pyrite, magnetite and some
chalcopyrite (Fig. 5). Pyrite and chalcopyrite replace
magnetite. The magnetite is predominantly cataclastic in
nature, but recrystallised ore also occurs. The gold is found
within pyrite and chalcopyrite. The gangue mineralogy
comprises quartz, K-feldspar, muscovite, biotite and carbonates
(calcite, dolomite and/or ankerite).
The mylonite zone is silicified at the contact with the
mineralised zone, and sulphide-rich parts are weathered.
Secondary goethite and malachite are common (Fig. 5). The
ore is structurally controlled by and confined to favourable
sites (sulphide-bearing zones) within the mylonite/
shear/fault zone.
The Attu gold prospect has returned reproducable gold
Table 1. Pb-isotopic ratios of magnetite, pyrite
and K-feldspar from the Attu gold prospect and its host rocks
Sample number Mineral 206Pb/ 204 Pb ± 2σ * 207Pb/ 204 Pb ± 2σ 208Pb/ 204 Pb ± 2σ r1 ** r2 †
Amphibolite and orthogneiss (host rocks)
446601 magnetite 14.631 0.007 14.642 0.009 44.688 0.033 0.961 0.939
446602 magnetite 15.051 0.014 14.752 0.015 36.702 0.040 0.969 0.942
446610 magnetite 17.540 0.051 15.361 0.046 37.613 0.112 0.977 0.988
446614 magnetite 17.002 0.025 15.225 0.024 38.086 0.061 0.976 0.967
Shear zone and mineralised rock
446616 magnetite 15.423 0.023 14.844 0.023 41.598 0.068 0.979 0.957
2000368 magnetite 15.286 0.009 14.832 0.010 41.201 0.034 0.962 0.936
481093 magnetite 14.247 0.042 14.625 0.044 41.821 0.130 0.982 0.960
446615 pyrite 14.241 0.009 14.587 0.010 42.001 0.035 0.963 0.934
481078 pyrite 14.447 0.011 14.633 0.012 40.805 0.039 0.967 0.925
446616 K-feldspar 15.123 0.008 14.893 0.010 36.451 0.029 0.958 0.932
* Errors are two standard deviations absolute (Ludwig 1990).
** r1 = 206 Pb/ 204 Pb versus 207 Pb/ 204 Pb error correlation (Ludwig 1990).
† r2 = 206Pb/ 204 Pb versus 208 Pb/ 204 Pb error correlation (Ludwig 1990).
56